In recent years, pixel sizes are rapidly becoming smaller, in response to demands for solid-state imaging elements that are smaller in size and include a larger number of pixels. Particularly, since there is a need to satisfy both the demands for smaller camera modules and a larger number of pixels, there are strong demands for pixel miniaturization in the imaging elements for portable telephone devices with camera functions. The pixel sizes are switching from 1.7 μm to 1.4 μm, and 1.1-μm pitch pixels are being developed.
However, as pixels become smaller, the tendency of the S/N ratio to be lower becomes inevitable. Particularly, since the absorption coefficient of single-crystal silicon with respect to visible light is low, the depth of each of the photodiodes for photoelectric conversions needs to be 3 μm or greater. In view of this, as the pixel sizes become smaller, an increase in aspect ratio of photodiodes in a pixel cross-section structure is inevitable. As a result, crosstalk noise due to obliquely incident light increases. Reducing the crosstalk noise is a critical issue in the trend toward pixel miniaturization. As described above, to prevent a decrease in the S/N ratio in pixel miniaturization, it is critical to reduce crosstalk noise.
When crosstalk noise is reduced, it is also critical to avoid an increase in photodiode noise. In decreasing photodiode noise in a solid-state imaging device, a so-called buried photodiode structure that has a high-density p+-type impurity diffused region in the vicinity of the interface between Si and a SiO2 layer is very effective to prevent the dark current generated in the interface state between Si and the SiO2 layer from flowing into photodiodes. Such a buried photodiode structure is now a standard feature in solid-state imaging devices.
However, when a buried photodiode structure is formed, it is difficult to form a very thin high-density p+-type impurity diffused region in the vicinity of the interface, and the thickness normally becomes 100 nm or greater.
To reduce crosstalk noise, on the other hand, photodiodes are effectively made thinner by increasing the light absorption efficiency in the photodiodes. This is because obliquely incident light can be prevented from entering adjacent pixels by reducing the invasion length of obliquely incident light.
It is known that forming strong electric fields localized by surface plasmon resonance is effective to reduce the thickness of each photodiode made of single-crystal silicon having a low light absorption coefficient.
However, the region in which strong electric fields are localized is located extremely close to the surface plasmon resonance structure, and is included in the high-density p+-type impurity diffused region in the vicinity of the interface in the above described buried photodiode structure. The signal electrons photoelectrically converted in the high-density p+-type impurity diffused region in the vicinity of the interface do not flow into the photodiodes, and therefore, are lost, without contributing to the sensitivity to light. As a result, an increase in the sensitivity is not achieved.
If the high-density p+-type impurity diffused region is not formed in the vicinity of the interface for the sake of increasing the sensitivity, the current generated from the interface level between Si and the SiO2 layer flow into the photodiodes, though the sensitivity becomes higher. In such a case, the noise increases, and the S/N ratio becomes poorer. As a result, high-sensitivity characteristics cannot be achieved.